Hdpe wear strips

[Oyster]

I have a design question,it might be a little long winded. I am building a fiberglass over cedar boat and would like to bolt some hdpe wear strips to the bottom of it. I looked around some and found out that 100 feet of hdpe with a 100 degree temp rise will expand 10 inches and 100 feet of fiberglass with a 100 degree temp rise will expand 1 inch. This data came from a pipe manufacture, so it may not be exactly what I am dealing with but should be fairly close.
The boat hull is 1/2 inch thick red cedar with 3 layers of 10 oz plain weave cloth with epoxy resin both inside and outside of the hull. I have bored 3/4 holes every 8 inches thru the hull before I glassed and filled them with resin thickened with wood flour along the length of the wear strip locates to be bored to 5/16 in the end. On the inside of the hull I will use 1 5/8 inch aluminum unistrut for stringers to back up were the strips will be.I thought I would bore the hdpe,hull and unisrut to accept 5/16 inch 304 ss cap screws. The hdpe strips will be 2 1/2 inches wide x 1 1/2 inches tall. Given the different expansion rates of these materials I have a worry of over stressing the hull or the cap screws that hold the strips on. So I guess the question is the strength of the hdpe enough to cause the hull panel to fail or shear the cap screws.?
Thank you for any input positive or negative.
Royce

[Wooden Boat Fittings]

Sorry, I can't answer your question directly. But it might pay to have a look at Post #26 in this thread -- the idea of screws in slotted holes suggests itself.

Mike

[JimConlin]

The other part of the analysis is how rubbery is the HDPE. I.e, if it were bolted firmly to something immovable, what force does it take to stretch/compress it from the length it wants to be?

[TerryLL]

UHMW is the usual choice for hull guards, bilge guards, rail caps, and other types of wear strips. It's used on all kinds of hull types, aluminum, steel, wood and FG, and I've never seen any indication of distortion due to longitudinal expansion. The stuff is very slippery, which is just what you want for bottom guards and has excellent impact resistance.


Edited to add:

I just did a quick search and found that the coefficient of thermal expansion is slightly lower for HDPE than it is for UHMW.

[Mrleft8]

Aluminum stringers and SS bolts?.....

[Woxbox]

I wish I could place it, but I'm sure I've seen that stuff bowing out leaving a gap behind it when installed like that. I think Jim's suggestion is right - that's it's more "rubbery" than the boat hull will be, which may or may not be a good thing.

[David G]

I wish I could place it, but I'm sure I've seen that stuff bowing out leaving a gap behind it when installed like that. I think Jim's suggestion is right - that's it's more "rubbery" than the boat hull will be, which may or may not be a good thing.

Yes, the differential coefficients of expansion is a very real issue. I've seen two effects. First, as Woxbox mentions, is the bowing out between fasteners. That's the immediate effect. The second is elongation of the fastener holes thru the plastic as it expands & shrinks repeatedly. Both can cause problems. I wish I had a nifty solution for you... maybe someone else will have solved this one. I've seen the same thing, btw, when people try and use Trex decking, and other 'plastic lumber' as rubrails and such. And, yes, I think you should reconsider mixing aluminum and stainless steel - esp. if this boat will ever see saltwater.

[Oyster]

The other part of the analysis is how rubbery is the HDPE. I.e, if it were bolted firmly to something immovable, what force does it take to stretch/compress it from the length it wants to be?

Thanks Jim ,
One chart I found reads 3480-6530 psi tensile strength , 2180-4350 yield strength for hdpe. I have not found anything out there about the properties of a cedar and glass composite panel

[Oyster]

Sorry, I can't answer your question directly. But it might pay to have a look at Post #26 in this thread -- the idea of screws in slotted holes suggests itself.

Mike

Thanks mike,
With a 100 degree differential, that is 2" of unrestrained growth in the length of the boat. I thought originally that i could gasket between the hdpe and the hull. I worry about the slots packing full of silt and sand as well as maintaining a water tight seal.

[Oyster]

Aluminum stringers and SS bolts?.....

I am guessing you are concerned with corrosion?

[Mrleft8]

I am guessing you are concerned with corrosion?

From what I've heard, Aluminum, SS and salt water mixed together are a bad idea.
Yes.... Corrosion would be my concern.

[Woxbox]

The screws will essentially weld themselves to the strip over time, and the only way to remove them will be to drill them out.
Personally, I'd just glue on strips of hardwood for this job. No fasteners, no holes through the hull. When (and if) they get badly chewed up, just plane them down and glue on another strip of whatever thickness is needed to replace the material that was removed.

[WI-Tom]

Personally, I'd just glue on strips of hardwood for this job. No fasteners, no holes through the hull. When (and if) they get badly chewed up, just plane them down and glue on another strip of whatever thickness is needed to replace the material that was removed.

I considered UHMW plastic for my keel strip on my whitehall, but in the end I think I've decided to go this route instead, for the same reasons.

Tom

[JimConlin]

If the shrink of the stick when cold is 10"/100', and the tensile modulus of HDPE is about 120K psi, I calculate a stress of about 1000 psi to pull it to its warm length.
I'm surprised it's that much. Did I do this right? Is the 10" expansion figure right?


I've got some HDPE bang strips on top of Damfino's port float for pulling the dinghy over. They're 6' long a screwed every 6" or so. I have not observed any buckling or pulling of the screws.

[AJZimm]

I see 2 possibilities:

1. Make the holes in the HDPE or UMHW oval in the first place and don't tighten down the bolts so much as to prevent the movement, and thereby allow the movement.

2. Steal a trick from the original railway rails - don't make the strip continuous the length of the boat. Cut it up into smaller strips with, say, an 1/8th of an inch gap between the ends, to allow the expansion longitudinally to take up with each smaller strip. It would still put some stress on the bolts and holes, but not as much on each bolt as with a continuous strip. An 1/8th inch gap shouldn't be enough to affect the function of the strip but if you are worried about stuff getting into the gap, you could half-lap the ends. At your stated expansion rate of 1" per 10' per 100 degrees, I reckon making each strip about a foot long, along with the aforementioned 1/8 gap, would take care of the problem

[TerryLL]

You might also consider doing it the old fashioned way, by using some type of tough hardwood for the rub strips.

[jerry bark]

tolman skiffs are routinely built using UHMW rub strips on the bottom. I have never heard of a problem with them yet. the typical process is to screw 10 foot strips every 6 inches and bedded in Sikaflex 291.

cheers
Jerry

[Oyster]

The screws will essentially weld themselves to the strip over time, and the only way to remove them will be to drill them out.
Personally, I'd just glue on strips of hardwood for this job. No fasteners, no holes through the hull. When (and if) they get badly chewed up, just plane them down and glue on another strip of whatever thickness is needed to replace the material that was removed.

Thanks woxbox,
I do not guess I under stand how the cap screws will weld themselves to the strip. It is looking like white oak might be less of a chore to do.
Royce

[Oyster]

If the shrink of the stick when cold is 10"/100', and the tensile modulus of HDPE is about 120K psi, I calculate a stress of about 1000 psi to pull it to its warm length.
I'm surprised it's that much. Did I do this right? Is the 10" expansion figure right?


I've got some HDPE bang strips on top of Damfino's port float for pulling the dinghy over. They're 6' long a
screwed every 6" or so. I have not observed any buckling or pulling of the screws.

Thanks Jim,
In no way shape or form am I even close to be qualified enough to discuss this ,but why let let that stop me ? I believe the 10" per 100' per 100 degrees is right. Now in reality temp differential is -60 to + 80, this is where I think cold spring comes into play, the temperature at which is drilled and bolted. There is 3.75 square inches of cross section of the strips so to my gut a 1000 lbs does not seem any too light
Royce

[Oyster]

I see 2 possibilities:

1. Make the holes in the HDPE or UMHW oval in the first place and don't tighten down the bolts so much as to prevent the movement, and thereby allow the movement.

2. Steal a trick from the original railway rails - don't make the strip continuous the length of the boat. Cut it up into
smaller strips with, say, an 1/8th of an inch gap between the ends, to allow the expansion longitudinally to take up with
each smaller strip. It would still put some stress on the bolts and holes, but not as much on each bolt as with a
continuous strip. An 1/8th inch gap shouldn't be enough to affect the function of the strip but if you are worried about
stuff getting into the gap, you could half-lap the ends. At your stated expansion rate of 1" per 10' per 100 degrees, I
reckon making each strip about a foot long, along with the aforementioned 1/8 gap, would take care of the
problem

Thanks Alex,
I think you are on to something with multiple strips,the issue I think that needs looked close at is keeping the silt from getting under or between the strips. good idea with lap joint
Royce

[Oyster]

You might also consider doing it the old fashioned way, by using some type of tough hardwood for the rub strips.

Thanks Terry,
I will be by myself in remote areas with this boat and would like to have any advantage I can to be able to push it off a gravel bar. A lot of the places I run are pool drop type rivers and creeks so it will be high grounded on occasion.any thing I can to break the fiction is a good thing

[Oyster]

tolman skiffs are routinely built using UHMW rub strips on the bottom. I have never heard of a problem with them yet. the typical process is to screw 10 foot strips every 6 inches and bedded in Sikaflex 291.

cheers
Jerry

Thanks Jerry,
Can you tell me what kind and size of the fasteners they use and what substrate they are going into or thru?
Royce

[TerryLL]

Thanks Terry,
I will be by myself in remote areas with this boat and would like to have any advantage I can to be able to push it off a gravel bar. A lot of the places I run are pool drop type rivers and creeks so it will be high grounded on occasion.any thing I can to break the fiction is a good thing

The UHMW is the slipperiest thing you can put on the bottom. It's used in sheet form to cover the bottom of drift dories so they slide over the rocks. Absolutely nothing sticks to it.

[JimConlin]

Thanks Jerry,
Can you tell me what kind and size of the fasteners they use and what substrate they are going into or thru?
Royce

I used panhead screws in counterbores.

[Gib Etheridge]

You might try giving the bottom a couple more coats of epoxy thickened with teflon powder. It's plenty slippery, but the powder softens the epoxy some. Easy to add more from time to time though.

[the_gr8t_waldo]

Have you considered "iron wood"?

[Woxbox]

Thanks woxbox,
I do not guess I under stand how the cap screws will weld themselves to the strip. It is looking like white oak might be less of a chore to do.
Royce

It's not technically a weld, it just feels like one when you try to disassemble it. Stainless screws are routinely used to attach hardware to aluminum spars, and in my experience, after some years they won't screw out no matter what you do.

As far as getting the boat back in the water, if there's serious risk of getting stuck, I suppose you could take along rollers and a winch or come-along as back ups.

[Oyster]

It's not technically a weld, it just feels like one when you try to disassemble it. Stainless screws are routinely used to attach hardware to aluminum spars, and in my experience, after some years they won't screw out no matter what you do.

As far as getting the boat back in the water, if there's serious risk of getting stuck, I suppose you could take along rollers and a winch or come-along as back ups.

Thanks woxbox,
The bolts would sandwich the hdpe,hull and stringer, so there would be no threads to seize up. As far as getting back to the water, I pack a rope come a long with a 300' line on it. When you are sure enough a ways from water I cut willows and use them as skids. Green willows are slicker than snot.
Thanks again for all the input.
Royce

[Oyster]

You might try giving the bottom a couple more coats of epoxy thickened with teflon powder. It's plenty slippery, but the powder softens the epoxy some. Easy to add more from time to time though.

Thanks gib,
Some of the creek bottoms are more like cobble stone than gravel,so as much for slickness the strips are to get the fiberglass another 1 1/2" higher to protect from impact damage. I ordered graphite powder for the fill coats but it did not come before I did them,so that is still an option if I buy more epoxy.
Royce

[CalebD]

Ipe or Ironwood.
Weight and strength where you want it.
Replaceable and not so expensive.

[Oyster]

Have you considered "iron wood"?

I have not considered iron wood or worked with before,can you tell me anything about it. Cost, rot resistance, hardness, availability ? What I know I can get my hands on without bad freight costs from the west coast are white oak, ash, hickory.

[JimConlin]

Graphite powder is sold to farmers for lubrication seed for planting machines.
McMaster has it, too.

[jerry bark]

Thanks Jerry,
Can you tell me what kind and size of the fasteners they use and what substrate they are going into or thru?
Royce

the screws recommended are 1 5/8 stainless screws that are made like a drywall screw. for the center strip the screws are driven into the epoxy fillet between the two sides of the bottom. the outer strips the screws go through the bottom plywood and into the stringers which are either 2 x 8 lumber (yellow pine in my boat, doug fir is used often) or LVL.

Jerry

[Oyster]

the screws recommended are 1 5/8 stainless screws that are made like a drywall screw. for the center strip the screws are driven into the epoxy fillet between the two sides of the bottom. the outer strips the screws go through the bottom plywood and into the stringers which are either 2 x 8 lumber (yellow pine in my boat, doug fir is used often) or LVL.

Jerry

Thanks Jerry,
A couple more things, what are the dimensions of the strips ? Do you notice the strips bowing out when it is out of the water and it is hot out ? If not does it seem as the sikaflex is bonded to the strips ?
Thanks again for all the info.
Royce

[Matti]

Just one idea; cut shoulders to the strips, if you have 2 1/2 " wide strip cut both side 1/4" shoulder, you have 2" wide wearing surface.
Put the strip on your boat bottom and make glass and epoxy groove on shoulders where to slide=use strip as mould. Bolt only one end.
If groove fails nothing happens, just you have loose strip.

[Oyster]

Just one idea; cut shoulders to the strips, if you have 2 1/2 " wide strip cut both side 1/4" shoulder, you have 2" wide wearing surface.
Put the strip on your boat bottom and make glass and epoxy groove on shoulders where to slide=use strip as mould. Bolt only one end.
If groove fails nothing happens, just you have loose strip.

Thanks matti,
I like the concept, I thought something similar. Form a channel section of aluminum with the flanges turned in parallel with the web and cut saw kerfs in either side of strip. Then bolt the channel to the hull and thead the strip into the channel and only bolt the strip at the bow. If putting these hdpe strips on gets to labor intensive or complicated I will probably use hard wood.
Thanks again
Royce
Royce

[Gib Etheridge]

I think in your situation that I would thru-bolt 1 1/2" X 2 1/2" oak to the bottom on the flat. Before bolting it in, bedded in caulk to exclude dirt and moisture, I would cut a dado with beveled edges on the underside, say 1 1/2" wide and 3/4" deep, like a full length dovetail slot, and slide in several pieces of UHMWPE, each one fastened at it's forward end only. If you leave a bit of space between each set of pieces you won't have to think about expansion or contraction forces.

[Oyster]

[QUOTE=Gib Etheridge;3313668]I think in your situation that I would thru-bolt 1 1/2" X 2 1/2" oak to the bottom on the flat. Before bolting it in, bedded in caulk to exclude dirt and moisture, I would cut a dado with beveled edges on the underside, say 1 1/2" wide and 3/4" deep, like a full length dovetail slot, and slide in several pieces of UHMWPE, each one fastened at it's forward end only. If you leave a bit of space between each set of pieces you won't have to think about expansion or contraction

Thanks gib,
I like this concept, this could be done also with aluminum by forming a channel with 135 degree bends instead of 90 degree bends.
Great thoughts by everybody
Royce

[jerry bark]

I do not have the strips on my tolman, I'm a rebel!

the typical dimensions are 3/4 x 3/4

Jerry

[the_gr8t_waldo]

firstly i have no idea what conditions you will be encountering, or where on the hull you plan on installing. hull design, availabilty of woods...on n on! ironwood, these days is a loose collection of very dense woods among 'em are american hornbeam, ipe, various olives,lignum vitae, and a whole bunch of others. i think it's only fair to think about the use of wear strips, is that they are replaced a few times over the boats life span. and since largly unseen, they can be left to the ravages of nature. personally i would much prefer much smaller wear strips. anyway, when people think of iron wood, most think of ipe. you should find it chealy enought if you go to a outdoor decking supply place and ask for the registered product, actually called "iron woods" that should help with your costs. good things about wood is that you probably don't have to consider it's expantion rates, and can get away with smaller lumber

[Oyster]

Thanks Jerry and Waldo
Royce

[Oyster]

thanks all,
i will post photos when i make a move on this.
royce

[Oyster]

After a fair bit of thought and study, I am thinking I will thru bolt 1" x 2" oak to the hull. Then screw the hdpe to the oak only every 3" staggered. does anybody see any issues with this method? In the name of not having the oak split it seems it should be flat sawn to me. Does that make sense to you fellas? The other thing is do you think I should bed the oak to the hull with anything,epoxy, 5200, lexel or any other product?
thanks for any input
royce

[Dan St Gean]

After a fair bit of thought and study, I am thinking I will thru bolt 1" x 2" oak to the hull. Then screw the hdpe to the oak only every 3" staggered. does anybody see any issues with this method? In the name of not having the oak split it seems it should be flat sawn to me. Does that make sense to you fellas? The other thing is do you think I should bed the oak to the hull with anything,epoxy, 5200, lexel or any other product?
thanks for any input
royce

Wear strips should be something easliy removable imho.

Dan

[Oyster]

Wear strips should be something easliy removable imho.

Dan

Thanks Dan
Royce

[Breakaway]

1. Make the holes in the HDPE or UMHW oval in the first place and don't tighten down the bolts so much as to prevent the movement, and thereby allow the movement

.

Plus 1. I have made numerous fittings for my fiberglass boat out of PE. This is what the King Starboard people tell you to do when fastening their product. They also strongly suggest through fastening, not screws.


Kevin

[Oyster]

[QUOTE=Breakaway;3334640].

Plus 1. I have made numerous fittings for my fiberglass boat out of PE. This is what the King Starboard people tell you to do when fastening their product. They also strongly suggest through fastening, not screws.

Ok Kevin ,
So starting from the inside of boat I have a aluminum c channel for stringers, then the 1/2" cedar and glass hull panel,then a 1" x 2" oak runner, then a 3/4" thick x 1 1/2" wide hdpe wear strip. Help me understand how you would fasten all this. I really do appreciate all the help you fellas offer.

[jerry bark]

the oak will hold screws just fine, screw to it. the starboard people do not want you to screw into starboard as it won't hold screws well.

I would bed the oak in sikaflex 291. I would also give the hole where you plan to bolt the oak a couple coats of epoxy on the inside of the hole and let cure before assembly.

Jerry

[Oyster]

Thank you Jerry,
Now I just have to wait for the unistrut to hit town.
Royce

[Oyster]

Ok then,
I am taking delivery on the oak for the rub strips this week. I will need to scarf them together, my jig is 16 to 1, I have some system 3 gel magic,some system 3 general porpoise, some titebond polyurethane. I will get one thru bolt thru the splice, so what glue would you guys use?